As for most neurodegenerative diseases, an intracellular inclusion (aggregate) unique to each disease is found in the brain of the neurodegenerative disease patient. Both neurofibrillary tangle observed in Alzheimer's disease (AD) and a pathological construct designated as “Lewy bodies” in Parkinson's disease (PD), are insoluble aggregates consisting of various kinds of proteins. Tau has been identified as the major component of neurofibrillary tangle; so have been α-synuclein as the major component of Lewy bodies; and TAR DNA-binding protein 43 kDa (TDP-43) as the major component of ubiquitin positive-intracellular inclusion that is observed in amyotrophic lateral sclerosis (ALS), respectively. While these proteins are present as soluble proteins in a normal subject, they are accumulated as insoluble and abnormal aggregates in the brain of a patient, the mechanism of which having remained unclear so far. Also, since correlations are observed between the sites where the aggregates appear and the sites of neuronal deficit (cell death), there is considered a mechanism that the aggregates appearing in the cell have cytotoxicity such that the deaths of neuronal cells eventually take place and that the onset of the diseases is thereby triggered. Accordingly, it is considered that elucidation of, for example, the mechanism of intracellular inclusion formation and the mechanism of induction of cell death by aggregates may contribute to the development of therapeutic agents for various neurodegenerative diseases.
For the purpose of the development of therapeutic agents, it is necessary to construct a system which can mimic the formation of distinctive intracellular inclusion of each disease in cultured cells. However, by simply expressing proteins for forming the aggregates in the cells, no aggregates observed in the brains of patients can be formed at all. In recent years, the inventors of the present application have established a method to efficiently introduce the protein aggregates prepared in vitro into cultured cells (patent document 1 and non-patent document 1). The inventors in the present application found that aggregates having a property and configuration similar to the intracellular inclusions observed in the brains of patients could be formed as a result of introducing the aggregates prepared in vitro into the cells in which the protein had been transiently expressed in advance. The formation of the aggregates is not observed at all when proteins of aggregates are merely transiently expressed in the cells. However, new aggregates are formed when the aggregates of the proteins prepared in vitro are introduced into the cells and then act as a nucleus i.e. seed of the aggregation.